Photonic crystals are materials having a periodic modulation in their refractive index (Yablonovitch, Phys. Rev. Lett., 58:2059, 1987), giving rise to a photonic band gap or stop gap, in which the propagation of electromagnetic waves within certain ranges of wavelengths is inhibited or restricted. The spectral positions of these bands are dependent on the distance between the periodic modulations in the crystal. The reflected stop band wavelengths can appear in the reflectance spectrum as a distinct reflectance peak known as a Bragg peak. The crystal may have a one-, two-, or three-dimensional (3-D) periodic structure.
A 1-D photonic crystal has a periodic multilayer structure, also referred to as a Bragg mirror. Such a structure has a reflectance peak for incident light in only one direction—perpendicular to the layers. A 2-D photonic crystal has a modulation in only two dimensions. Such a structure has a reflectance peak for incident light in two directions—along the plane of the repeating units. A 3-D photonic crystal has an ordered periodicity in all three dimensions. Such a structure has reflectance peaks for incident light in all directions. Methods for manufacturing these photonic crystals include etching, and in the case of a 3-D photonic crystal, self-assembly of microparticles. Such photonic crystals may also be inverse crystals, in which the periodic structure of the crystal comprises a periodic array of voids.
An inverse photonic crystal can be made using a templating strategy. For 3-D inverse photonic crystals, 3-D photonic crystals formed by microparticles are used as a template for an infiltrating material. When the template is removed, the result is an inverse photonic crystal having an ordered array of voids. Such a templating strategy is disclosed in U.S. Pat. No. 6,261,469. The photonic crystal disclosed in this reference is in block form, which may not be suitable in many applications.
Because of the sensitivity of a photonic crystal, slight changes in the refractive index or lattice spacing result in detectable changes in the reflected light. This is useful where the refractive index or the lattice spacing of the photonic crystal is modulated in response to external stimuli or can be controlled. Examples of such applications are given in U.S. Patent Application Publication No. 2004/0131799, PCT Application Publication No. 2008/098339, and U.S. Patent Application Publication No. 2009/0034051. Deformable photonic crystal are also known, comprising non-close-packed spheres embedded in an hydrogel or elastomer matrix, for example as discussed in U.S. Pat. No. 6,544,800 to Asher, U.S. Pat. Nos. 5,266,238 and 5,368,781 to Haacke et al., by Holtz et al. in Nature 389:829-832, by Foulger et al. in Advanced Materials 13:1898-1901, by Asher et al. in Journal of the Material Chemical Society 116:4997-4998, and by Jethmalani et al. in Chemical Materials 8:2138-2146.
Examples of photonic crystal structures that can respond to external stimuli include colloidal photonic crystals in the form of optical films (Busch et al., Phys. Rev. E, 58:3896, 1998; Xia et al., Adv. Mater., 12:693, 2000). The reflectance wavelength ranges of these materials are highly sensitive to changes in the external environment, optical characteristics, or the structure of the photonic crystal.
The use of 3-D photonic crystals as color particles, flakes or pigments has been previously proposed. An advantage of photonic crystal pigments, flakes or particles is that they can be dispersed in inks or paints, facilitating application of a photonic crystal film to a surface. Coatings of photonic crystal pigments, flakes or particles may reflect wavelengths in a wider viewing angle, due to different or randomized orientations of the particles in an ink or paint. Use of such photonic crystal particles in inks or paints have been suggested. U.S. Pat. No. 6,756,115 describes the use of 3-D colloidal photonic crystal particles for color pigments. U.S. Patent Publication No. 2006/0288906 describes a process for producing photonic crystal particles usable as pigments or in inks. PCT Publication No. 2004/104115 describes a multi-layer coating for a substrate surface comprising a layer having 3-D photonic crystals as pigments. These applications provide inks, pigments and coatings having an opalescence effect. The photonic crystals in these inks, pigments and coatings are designed to reflect a certain desired wavelength of light. Multilayer diffractive pigments based on diffraction gratings have also been suggested, for example in U.S. Pat. No. 6,841,238.
Other pigments or inks having optical effects have also been suggested. U.S. Pat. No. 6,695,905 suggests use of optically variable pigments based on Fabry-Perot resonance. U.S. Pat. No. 6,749,777 suggests use of diffractive pigment flakes for optical effects.
It is desirable to have a photonic crystal particle, ink or pigment in which the reflected wavelength may be tuned, so as to be responsive to external stimuli or to be controllable. It is also desirable to have a photonic crystal pigment, flake or particle that can be used in standard inks, coatings and paints, in a variety of applications.